1. Field of the Invention
The present invention relates to an adhesive composition, an adhesive varnish, an adhesive film and a wiring film; more specifically to an adhesive composition, an adhesive varnish, an adhesive film and a wiring film which excel in adhesion property, preservation stability and handleability.
2. Related Art
Since electric and electronic devices have become more space-saving and lighter in recent years, an internal wiring material is required to enable a high density wiring through a micro wiring and a thinner body. Further, more and more devices are using lead-free solder instead of conventional lead solder. As a result, the internal wiring material is required to be more heat-resistant.
An insulating layer of the above wiring material basically comprises a base material film and an adhesive layer (see e.g. JP-A H05-29399). As the base material film, e.g., an organic insulating film comprising a heat-resistant film such as polyimide, polyetherimide, polyphenylene sulfide, polyether ether ketone, and a composite heat-resistant film such as an epoxy resin glass cloth, an epoxy resin polyimide glass cloth is disclosed. As an adhesive layer, an adhesive including a polyamide resin and an epoxy resin is disclosed.
However, the adhesive disclosed in JP-A H05-29399 has a disadvantage that the preservation stability is low due to high reactivity between an amino group existing in the structure of polyamide resin and an epoxy resin. An adhesive comprising a phenoxy resin having an epoxy group at ends, acrylic rubber and a curing agent is proposed to solve the above disadvantage (see e.g. JP-A 2004-136631). As the phenoxy resin, e.g. bisphenol A type, bisphenol F type, bisphenol AD type, bisphenol S type or copolymerization type of bisphenol A and bisphenol F are disclosed. Although the adhesive disclosed in JP-A 2004-136631 is produced by compounding a phenoxy resin, which is to have a relatively high adhesion property, there is a disadvantage in that adhesion force is as weak as 0.5 kN/m and that the soldering heat resistance is slightly low level of 260° C.
As a method for solving the above problems, an adhesive including a thermoplastic polyurethane resin with a mass average molecular weight of 80,000 to 800,000, an epoxy resin, an epoxy resin curing agent is disclosed (see e.g. JP-A 2010-150437).
Although a normal polyurethane resin has a disadvantage in preservation stability because of its high reactivity with an epoxy resin, JP-A 2010-150437 discloses that using a polyurethane resin having a specific molecular weight range improves the preservation stability. Adhesion force thereof is 1.1 to 1.7 kN/m.
Further, it is disclosed that the soldering heat resistance of an adhesive including a polyurethane resin, an epoxy resin and a novolac resin with a specific structure is 300° C. (see e.g. JP-A 2010-143988). However, the polyurethane resins used in JP-A 2010-150437 and JP-A 2010-143988 have been generally known to decopolymerize at a temperature of 200° C. or higher. Since the heat resistance of polyurethane is generally to be 80 to 100° C., there are some concerns about applying the adhesive including a polyurethane resin to the field of industrial and automobile electronic devices, which demands high heat resistance.
Further, an adhesive including a (meth)acrylic-modified phenoxy resin and an epoxy resin, and similarly an adhesive including a (meth)acrylic-modified phenoxy resin, an urethane acrylate oligomer and a silane coupling agent are respectively disclosed (see e.g. JP-A 2001-262111 and JP-A 2008-258607). The adhesive including an epoxy resin disclosed in JP-A 2001-262111 has a disadvantage that adhesion force is as low as 0.6 kN/m despite superior heat resistance. Further, as for the adhesive including an urethane acrylate and a silane coupling agent disclosed in JP-A 2008-258607, adhesion force with a polyimide film, which has a high heat resistance despite its superior adhesion force with a metallic conductor, is not thoroughly taken into consideration.
Moreover, it is important in handling the adhesive film that a curl is as small as possible, and that there is no tuck on an adhesive layer.